A commonly known multilayer polyolefin separator which may be used in a lithium ion rechargeable battery is a dry process, uniaxially stretched, polyolefin trilayer (or tri-layer) separator produced by Celgard, LLC of Charlotte, N.C., and includes three layers of a polyolefin separator membrane or film configured as polypropylene/polyethylene/polypropylene (PP/PE/PP trilayer), where an inner polyethylene (PE) microporous membrane is sandwiched between two outer layers of polypropylene (PP) microporous membrane. The inner PE layer in the multilayer microporous separator may function as a thermal shutdown layer in the event of a thermal run away event. The use of polypropylene as the outer layers in such a trilayer battery separator structure may provide a higher mechanical and thermal strength. In some instances, use of polypropylene as the outer layers in a trilayer battery separator may be a preferred configuration. U.S. Patent Publication No. 2007/0148538 proposes a multilayer configuration of ‘polypropylene/polyethylene/polypropylene’ (PP/PE/PP) as a microporous trilayer separator where polypropylene may be used as the outer or exterior layers or films due to its higher tensile strength and higher melt temperature and polyethylene may be used as the inner polyethylene layer or film due to its lower melt temperature and thermal shutdown function. U.S. Pat. Nos. 5,952,120, 5,691,077, and 8,486,556 and U.S. Patent Publication Nos. 2014/079980, and 2008/118827 disclose various methods for making dry process multilayer microporous separators where PP may be used as the outer layers and PE may be used as the inner shutdown layer in a trilayer PP/PE/PP microporous separator for a lithium ion rechargeable battery.
FIG. 1 presents a schematic for a known method of manufacturing a trilayer PP/PE/PP microporous separator as described in US Patent Publication No. 2007/0148538. After extrusion, nonporous PP and PE layers or films are stacked in a trilayer configuration and laminated using heat and pressure to form a nonporous PP/PE/PP trilayer precursor. Subsequent steps of annealing and machine direction stretching of the nonporous PP/PE/PP trilayer precursor produce a uniaxial stretched PP/PE/PP trilayer microporous separator. The lamination step may be described as a bonding step which may be performed with heat and pressure using nip rollers. Lamination and/or bonding may commonly be used to join two polymer materials together with heat and pressure.
The thickness of a multilayer microporous separator plays an important role in the design of a lithium ion battery. A microporous separator membrane or separator which has a thickness less than 10 μm may be desirable because it may take up less space inside a battery and may allow for more electrode active material to be packed in a battery cell to produce a higher energy density and higher rate capability battery.
Furthermore, thinner microporous membranes may provide a preferred microporous substrate for polymeric-ceramic based coatings. U.S. Patent Publication No. 2014/0045033 discloses aqueous polymeric-ceramic based coatings that range from 4-7 μm in thickness which may be coated onto a 12-18 μm thick PP/PE/PP microporous membrane. The total thickness of the coated PP/PE/PP membrane may range from 16 to 25 μm.
For at least certain battery applications or technologies, there is still a need for thinner, stronger, more uniform, better performing membranes, dry process membranes, separator membranes, coated membranes, membranes with unique structures, membranes with enhanced performance, membrane separators, battery separators, shutdown separators, and/or batteries or cells including such membranes or separators, and/or methods of making such membranes, separators, cells, and/or batteries, and/or methods of using such membranes, separators, cells, and/or batteries. There is a need for a multilayer shutdown microporous membrane that has a thickness less than 10 μm for use as a battery separator and/or as a microporous substrate for polymeric-ceramic based coatings to form a coated battery separator. In addition, there is a need for a multilayer shutdown microporous membrane with a thickness less than 10 μm which may be easily coated with a polymeric-ceramic based coating where the coating has excellent adhesion to the membrane and excellent adhesion to an electrode. Furthermore, there is a need for a multilayer shutdown microporous membrane with a thickness less than 10 μm which may be coated with a polymeric-ceramic based coating where the coating thickness may be less than 7 μm. Furthermore, there is a need for a multilayer shutdown microporous membrane with a thickness less than 10 μm which has excellent machine direction (MD) and transverse direction (TD) tensile strength and that can be easily coated with a polymeric-ceramic coating.